agent: Improve initial file search quality (#29317)

This PR significantly improves the quality of the initial file search
that occurs when the model doesn't yet know the full path to a file it
needs to read/edit.

Previously, the assertions in file_search often failed on main as the
model attempted to guess full file paths. On this branch, it reliably
calls `find_path` (previously `path_search`) before reading files.

After getting the model to find paths first, I noticed it would try
using `grep` instead of `path_search`. This motivated renaming
`path_search` to `find_path` (continuing the analogy to unix commands)
and adding system prompt instructions about proper tool selection.

Note: I know the command is just called `find`, but that seemed too
general.

In my eval runs, the `file_search` example improved from 40% ± 10% to
98% ± 2%. The only assertion I'm seeing occasionally fail is "glob
starts with `**` or project". We can probably add some instructions in
that regard.

Release Notes:

- N/A

assets/prompts/assistant_system_prompt.hbs

@@ -31,6 +31,9 @@ If appropriate, use tool calls to explore the current project, which contains th
 - When looking for symbols in the project, prefer the `grep` tool.
 - As you learn about the structure of the project, use that information to scope `grep` searches to targeted subtrees of the project.
 - Bias towards not asking the user for help if you can find the answer yourself.
+{{! TODO: Only mention tools if they are enabled }}
+- The user might specify a partial file path. If you don't know the full path, use `find_path` (not `grep`) before you read the file.
+- Before you read or edit a file, you must first find the full path. DO NOT ever guess a file path!
 
 ## Fixing Diagnostics